Three brain areas the hippocampus, the prefrontal cortex, and the amygdala are particularly susceptible to pathological changes in size and function.

A hyperactive amygdala, along with abnormal activity in other regions of the brain, leads to disrupted patterns of sleep and physical activity, as well as irregular patterns of secretion of hormones and other chemical mediators that control many of the body's structures.

On the other hand, rather than atrophying, the amygdala, the seat of emotional memories, becomes hyperactive both in post traumatic stress disorder and depressive illness, and there is evidence from animal studies of a hypertrophy of nerve cells after repeated stress.